In general, a transparent electrode is variously used for electrodes for a variety of displays, a photoelectric transformation element like a solar cell, a touch panel, etc., and is fabricated by forming a transparent conductive thin film on a transparent substrate such as glass, a transparent film, etc. As a transparent conductive material, tin (Sn) doped indium oxide (ITO) is mainly used at present, which is known to have high transparency and low resistivity (1×10−4˜2×10−4 Ωcm).
There are various methods of fabricating such a transparent conductive thin film, including a vacuum-deposition method, such as sputtering, chemical vapor deposition (CVD), ion beam deposition, pulsed laser deposition, etc., and a wet method, such as spray coating, spin coating, and dip coating. From among the various methods, a vacuum-deposition method, such as sputtering, is more preferred. In the vacuum-deposition method, the use of plasma allows a film having high particle energy to be grown, and thus, it is possible to obtain a film having good quality and higher density than other methods. In addition, even at low temperatures without additional heat treatment, a high quality of film can be obtained.
As a market for a flat display becomes wider, a demand for ITO has been recently rapidly increased. However, indium has instability in demand/supply due to its high cost and shortage, and is harmful to the human body. Therefore, a transparent conductive material of low cost, which can be substituted for ITO, is required to be developed.
As such substitutes, tin oxide (SnO2), zinc oxide (ZnO), etc. have been considered. Specifically, a fabrication of a conductive thin film having low resistivity (2×10−4˜3×10−4 Ωcm) was tried by doping aluminum (Al) on zinc oxide. Zinc oxide (ZnO) is a semiconductor material having a wide band gap (˜3.3 ev) and is known to have high transmittance (85% or more) and low resistivity through doping. Also, doped zinc oxide is relatively cheap and harmless to the human body, and thus has been noticed as a substitute material for ITO. At present, a transparent electrode material using zinc oxide (ZnO) added with aluminum (Al), gallium (Ga), silicon (Si), and/or indium (In) has been mainly researched, but such a material has a problem in that electrical conductivity is lower than that of ITO.
Usually, since resistivity of a transparent conductive film having a certain thickness is in inverse proportion to concentration of electrons and mobility (ρ=1/(eμN), e: electric charge, μ: mobility, N: concentration of electrons), in order to decrease the resistivity (ρ) of a transparent electrode, the concentration or mobility of electrons is required to be increased. In order to easily increase the concentration of electrons, a method of increasing the amount of dopant included in a sputtering target may be used. However, when the concentration of electrons is more than a certain level (N>1×1020 cm−3), the mobility is decreased by collision between electrons and dopant functioning as a scattering center (when N<1×1020 cm−3, the mobility depends on the scattering on grain boundary). In other words, the increase in the concentration of electrons, which is obtained by increasing the amount of dopant included in the sputtering target, is accompanied with the decrease in the mobility, and accordingly, the resistivity is maintained as it is, or even increased. Such a relationship between the concentration of electrons and the mobility is experimentally known by many researchers. Accordingly, the conventional method has a limitation in improving an electrical property of a transparent conductive film.
Also, in depositing a film by using sputtering, the deposited film is damaged by high energy of sputtering particles, thereby causing unevenness of resistivity. Such unevenness of resistivity may be solved by changing the position of a substrate or the position of a magnet within a sputter device during a process of depositing a film, but this method cannot fundamentally solve the problem.